1. Field of the Invention
The present invention relates to a light-emitting element which includes a light-emitting organic compound or inorganic compound and which emits light when a voltage is applied. In particular, the invention relates to a light-emitting element which exhibits white light emission and to a light-emitting device employing the light-emitting element which exhibits white light emission.
2. Description of the Related Art
In recent years, there has been extensive research and development of light-emitting elements which employ a light-emitting organic compound, which are a kind of light-emitting element. In a common structure of such a light-emitting element, a layer including a light-emitting organic compound or inorganic compound (hereinafter referred to as a light-emitting layer) is interposed between a pair of electrodes, and when a voltage is applied to the element, electrons and holes are each injected and transported from the pair of electrodes to the light-emitting layer. When those carriers (electrons and holes) recombine, an excited state of the light-emitting organic compound or inorganic compound is formed, and when the light-emitting organic compound or inorganic compound returns to a ground state, light is emitted.
Further, kinds of excited state that can be formed by an organic compound are a singlet excited state and a triplet excited state. Light emission in the case of a singlet excited state is referred to as fluorescence, and light emission in the case of a triplet excited state is referred to as phosphorescence.
Such a light-emitting element is usually formed of thin films which have an approximate thickness of submicrons to several microns. Therefore, they can be manufactured to be thin and light, which is a large advantage. Further, such light-emitting elements also have an advantage in that the period of time from when the carriers are injected until light is emitted is microseconds at the most, so they have a very high response speed. Moreover, because sufficient light-emission can be obtained with a direct current voltage of approximately several to several tens of volts, power consumption is also relatively low. Due to these advantages, the above-described light-emitting element is attracting attention as a next-generation flat-panel display element.
Further, because the pair of electrodes and the light-emitting layer are formed as films in such a light-emitting element, surface light emission can easily be obtained by forming a large-area element. Since this is a feature which is hard to obtain in light sources such as incandescent lamps and LEDs (point light sources) or in fluorescent lamps (line light sources), the above-described light-emitting element has a high utility value as a light source such as a lighting device.
Considering these fields of application, it can be said that the development of a light-emitting element such as that described above which is a white light-emitting element is an important subject. If a white light-emitting element with sufficient luminance, light-emitting efficiency, element life, and chromaticity can be obtained, by combining it with a color filter, a quality full color display can be manufactured, and further, application to white light sources such as backlights and lighting devices can be also be considered.
Rather than being light-emitting elements which exhibit white light emission which has peaks in the wavelength ranges of red, green, and blue (the three primary colors of light), most white light-emitting elements are light-emitting elements which exhibit white light emission which is a combination of complementary colors; for example, blue light emission and golden yellow light emission (such light-emitting elements will hereinafter be referred to as 2-wavelength-type white light-emitting elements). For example, see Non-Patent Document 1.    [Non-Patent Document 1] Chishio Hosokawa (and 7 other authors), SID '01 Digest, 31.3 (pp. 522-525), 2001.
In Non-Patent Document 1, white light-emission is obtained by stacking two light-emitting layers which have a complementary color relationship, such that they are in contact with each other. Such a 2-wavelength white light-emitting element has high light-emitting efficiency, and can have a relatively long element life. In Non-Patent Document 1, values of an initial luminance of 400 cd/m2 and a luminance half-life of 10,000 hours were obtained.
However, while a white color which is good from a CIE chromaticity coordinates point of view can be obtained with a 2-wavelength white light-emitting element, the light emission spectrum of the white color is not continuous, and only has two peaks, which have the complementary color relationship. Accordingly, it is difficult to obtain broad white light which is close to natural light. Further, if the spectrum of one of the complementary colors is dependent on current density or lighting time and varies, the chromaticity tends to deviate greatly from white. Further, if the spectrum of one of the complementary colors varies, in the case of a full color display in which the element is combined with a color filter, transmission spectrums of the color filters for red, green, and blue do not match the light emission spectrum of the element, so it is difficult to produce desired colors.
Meanwhile, research and development of white light-emitting elements which are not 2-wavelength white light-emitting elements such as those described above, but rather have a light emission spectrum with peaks in the wavelength ranges of red, green, and blue (hereinafter referred to as 3-wavelength-type white light-emitting elements) is also underway (e.g., see Non-Patent Document 2 and Non-Patent Document 3). Non-Patent Document 2 describes a structure in which three light-emitting layers, a red one, a green one, and a blue one, are layered. Non-Patent Document 3 describes a structure in which light-emitting materials which exhibit red, green, and blue light emission are included in the one light-emitting layer.    [Non-Patent Document 2] J. Kido (and 2 other authors), Science, vol. 267, pp. 1332-1334, 1995.    [Non-Patent Document 3] J. Kido (and 2 other authors), Applied Physics Letters, vol. 67 (16), pp. 2281-2283, 1995.
However, these 3-wavelength white light-emitting elements are inferior to 2-wavelength white light-emitting elements in the areas of light-emitting efficiency and element life, and greater improvements of them are necessary. Further, it is known that in many cases, the spectrum of elements such as the one described in Non-Patent Document 2 is dependent on flowing current density and varies, so stable white light cannot be obtained.
Further, attempts to develop a white light-emitting element which approach the task from a different angle to that of Non-Patent Documents 1, 2, and 3 have also been carried out (e.g., see Patent Document 1 and Patent Document 2). Patent Document 1 and Patent Document 2 describe attempts to obtain a high current efficiency (the luminance which can be obtained with respect to a given current density) by stacking a plurality of light-emitting elements in series and combining the light emission from each of the light-emitting elements. Further, it is disclosed that by stacking light-emitting elements with different light-emission colors in series, a white light-emitting element can be obtained.    [Patent Document 1] Japanese Patent Laid-Open No. 2003-264085    [Patent Document 2] Japanese Patent Laid-Open No. 2003-272860
However, in the methods disclosed in Patent Documents 1 and 2, in order to obtain a 3-wavelength white light-emitting element, for example, it is necessary to layer three elements in series. That is, in order to manufacture a white light-emitting element having a spectrum which covers a broad wavelength range (a white light-emitting element in which a plurality of different light emission colors are mixed), the number of light-emitting elements which are stacked in series greatly increases accordingly, and the driving voltage multiplies. Further, because many light-emitting elements are stacked in series, the total stacked film thickness increases, so light emission tends to be affected by optical interference. Therefore, it is difficult to adjust the light emission spectrum.
As mentioned above, while conventional 2-wavelength white light-emitting elements have high light-emitting efficiency and a long element life, they have a problem in that their spectrum does not cover some parts of a broad wavelength range. Further, accompanying this problem, the chromaticity of their white color tends to change over time. Moreover, in addition to the fact that conventional 3-wavelength white light-emitting elements have low light-emitting efficiency and a short element life, they have a problem in that the shape of their spectrum tends to be dependent on current density. Further, it is not practical to try to obtain a white light-emitting element having a spectrum which covers a broad wavelength range using the methods disclosed in Patent Documents 1 and 2, because when these methods are used, the number of light-emitting elements which are stacked in series greatly increases and driving voltage greatly increases.
Further, the present applicant has disclosed a 4-wavelength-type white light-emitting element in Patent Document 3.    [Patent Document 3] Japanese Patent Laid-Open No. 2006-12793